A quick checklist of possible solutions to what can be triggering the popup

NoScript, Ghostery, Disconnect, Blur and Privacy Badger were reported by users to be causing this.

Even if AdBlock is disabled or the site is whitelisted - it may be filtered through another adblocking extension. Try disabling all extensions and enable them back one by one to find what is causing this.

Antivirus programs are sometimes blocking access to certain ressources. Avira Antivirus has been reported to trigger this popup. Disable temporarily your antivirus to see if it's causing the problem.

Try launching your browser in safe mode. Here is how to enable it for FireFox.

If you're using FireFox - in Tools / Options / Privacy / History - if "never remember history" is selected - this is also reported to be causing trouble.

If you have modified your hosts file or if you have a firewall program - check them out and make sure they're not blocking access to Google's ads servers.

If nothing from the above helps - get in touch and I'll help you fix it right away!